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A Near-Global Event Alley et al., Weakened Asian Monsoon (from speleothems of Dongge Cave, southern China) Southward shift of the ITCZ inferred from the Cariaco Basin record European region experienced strong cooling Sahara experienced drying Cooling in North America, with drying in the US Great Plains

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Questions to Address Is this a synchronous event? –Evidence from ice cores What triggered the event? –Freshwater forcing What was the impact of the event? –THC sensitivity Uncertainties? –Lack of uniform record

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Same Trigger as YD? Younger Dryas may have been triggered by an outburst of waters from a large ice- dammed lake and sustained by the redirection of meltwater from the Mississippi to the St. Lawrence Valley (Clarke, 2003) 10

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Outburst Northward outburst ka BP Marine geophysical surveys support high rates of water discharge in Hudson Bay associated with one or more outburst floods –megaripple sand-wave bed forms –arcuate scours on floor of Hudson Bay (Clarke, 2003) Hematite-rich rocks from the northern part of Hudson Bay are thought to be the source of red clay marker beds within Hudson Strait (Keigwin et al., 2005) Oldest marine mollusk around Hudson Bay ~8.45 ka Modern outburst analogs found in Iceland but not so big (Clarke, 2003) 14

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Mechanics Clarke et al., Water establishes a subglacial path Conduit grows by melting, stays open as long as water pressure exceeds overburden pressure Following an outburst, flood channel either remains open (smaller diameter) or reseals so that lake level rises until a subsequent flood is released (Clarke, 2004)

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Marine Cores: GGC26 Increase in abundance of N. pachyderma Carbon isotope ratios of C. wuellerstorfi low enough to indicate significantly decreased NADW production at several times in the Holocene, including 8.2 ka. Not all cores show same trends 17 Keigwin et al., 2003

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GCM Suggests Sensitivity General circulation model (GCM) by Geophysical Fluid Dynamics Laboratory in Princeton suggest NADW circulation is highly sensitive to freshwater forcing Some models project enhanced freshwater fluxes to North Atlantic will slow or stop deep water formation if maintained long enough Sv delivered to the Labrador Sea sufficient to stop convection in one model (Alley et al., 1997) NADW circulation winds down with an input of less than 0.06 Sv into the catchment area of the North Atlantic (Rahmstorf, 1995) May collapse if a certain threshold is exceeded and can show hysteresis behavior 22

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Wiersma Results Freshwater forcing in Labrador Sea produced a temperature anomaly over central Greenland in agreement with that observed during the 8.2 ka event Detectable temperature response to a freshwater forcing is not synchronous, lags mostly in the order of decades –Delayed response over Greenland of 30 years –Simulation suggests a delay of more than 50 years of detectable cooling over Asia 25

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Results (cont.) Lag due to an initial decadal warming –brief westward shift of deep-water formation from just south of Svalbard to north of Iceland –brings additional heat to Greenland (Wiersma, 2011, Renssen, 2001) Substantial increase in sea-ice coverage, with most of the Nordic Seas and the Denmark Strait becoming perennially ice covered (Renssen, 2001) Sea-ice cover causes a considerable cooling of the lower atmosphere over the Nordic Seas and adjacent landmasses (Renssen, 2001) 26

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Relevance Freshwater fluxes of similar magnitude may occur in future –Global warming of 3°C in response to doubling atmospheric CO 2 could increase total freshwater flux from Greenland ice sheet by 0.02 Sv and maintain the level over centuries (Alley et al., 1997) –Enhanced high latitude precipitation and sea ice melting in response to warming might cause an increase of similar magnitude in freshwater flux to North Atlantic (Alley et al., 1997) Freshwater flux at the right time, right place could trigger abrupt climate change (Alley et al., 1997) 29